The aperture blades, to be sure, are wide open when PDAF occurs. Marianne contends that the effective light cone used by the AF sensors is stopped down to an effective f/5.6. It is a very, very different thing!

I have Sigma AF500mm f7.2 and is still AF even with 1.4TC on it even on D70 in a basement with only one 60W light bulb very slow but it dose.